Earth May Not Have Needed Moon for Life

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Scientists have long believed that without the moon's stabilizing
gravitational influence, variations in Earth's tilt would have
caused climate change too dynamic for complex life to evolve. Not
so, concludes a new study that has implications for understanding
conditions for life elsewhere in the solar system.

The study sprang from the ongoing Kepler Telescope mission to
find Earth-like planets circling in habitable zones around other
stars in the Milky Way.

"We were wondering 'Do we really have to find a moon or not?'
around potentially habitable worlds, planetary scientist Jason
Barnes, with the University of Idaho, told Discovery News.

Previous studies showed that without the steadying gravitational
influence of a large moon, Earth's tilt would shift by as much as
about 85 degrees every 100,000 years or so, alternatively
freezing and baking the planet's poles. Scientists believe a
stable climate spanning about 500,000 years was necessary for
complex life to blossom on Earth.

A new computational analysis, however, shows that a moonless
Earth would still have swings in its tilt but the influence of
Jupiter and other factors would limit the variations to about 10
degrees in either direction.

"On the moon we can find important evidence and clues of what
happened to not only to the moon, but also to the Earth-moon
system over the last 4.5 billion years," said Vondrak, a
planetary scientist with NASA's Goddard Space Flight Center in
Greenbelt, Md.

The study also showed that if Earth revolved around the sun in
the opposite direction, called a retrograde orbit, it wouldn't
need a moon at all to have a climate about as stable as it has
today. Likewise, a Jupiter about half the distance to Earth as
its present location would have had a similar steading hand,
Barnes added.

The findings are causing extrasolar planet hunters to revise
their thinking on what constitutes a habitable planet.

"We think that at least 80 or 90 percent of planets out there
statistically won't even require a moon" to have a stable
climate, Barnes said.

Location is key. In our own solar system, Mars shows evidence of
extreme climate change, the result, scientists believe, of a
rotational tilt that flips between zero and 60 degrees over time.
A big moon likely could have helped stabilize Mars' orbit, but
the planet has just two small moons, most likely captured
asteroids, that don't have much gravitational muscle.

Other factors impacting a planet's climate and suitability for
life include its star's age, composition and the size and
location of sibling planets in the system.

"It's a very complex problem for sure and we're not anywhere near
solving it, but we're making positive steps as we slowly evaluate
each of these conditions and discover what constraints they
really place on whether life can exist there," Barnes said.

The research appears in this month's Astrobiology
magazine and is pending publication in the journal
Icarus.